1. Field of the Invention
The present invention concerns some improvements to compressors.
2. Description of Related Art
In particular, the present invention concerns a compressor for compressing gases of the type comprising at least one compressor element with a gas outlet and a gas inlet, as well as a sensor to determine the outlet temperature in the gas outlet, a sensor to determine the rotational speed of the compressor element, a motor with an electronically adjustable speed driving this compressor element, and finally a control device for said motor.
It is known that such compressors can operate within a specific maximum speed range of the number of revolutions, between a maximum and a minimum number of revolutions which depends among others on the mechanical limitations of the rotating parts, whereby irrevocable damage can be caused to the compressor in case the number of revolutions exceeds said speed range.
The speed range is usually characterised by the ratio between the maximum number of revolutions and the minimum number of revolutions, whereby the value of this ratio is typically situated around 3.2.
It is also known that a further restriction of the speed range is imposed by a phenomenon caused by a drastic output reduction of a compressor in the high and low speed range, as a result of which, as the rotational speed of the compressor comes closer to the aforesaid maximum or minimum number of revolutions, the temperature of the compressed gas can raise to such an extent that the coatings of the compressor element and of the downstream parts of the compressor may be damaged by the heat. In practice, this occurs when the temperature on the outlet of the compressor element exceeds an admitted maximum critical threshold value of 260 to 265° C.
In order to restrict the influence of the output reduction and to prevent the temperature on the outlet of the compressor element from rising above the aforesaid threshold value, it is important to further restrict the above-mentioned admitted speed range, all the more when the circumstances having an influence on the temperature rise are more adverse, namely in case of high ambient temperatures, when the finishing quality of a new compressor is not so good, in case of increasing wear of a used compressor and the like.
Compressors of the above-mentioned type are already known which are equipped with a fixed speed limiter, in particular a speed limiter with a fixed minimum and maximum threshold value for the rotational speed, whereby the most adverse circumstances are taken as a basis to determine said fixed threshold values, namely for a compressor with a minimum production quality, a certain degree of wear and operating at a maximum admitted ambient temperature.
A disadvantage of such known compressors with a fixed speed limiter is that the set speed range which is determined on the basis of a worst case scenario, assuming the most adverse circumstances, is in fact too restricting for circumstances which are less adverse, such as for example in case of lower temperatures, allowing in principle for a higher speed range without exceeding the aforesaid maximum critical threshold value of the temperature on the outlet of the compressor element. This implies that the capacity of such a compressor cannot be used to the full as far as the delivered gas flow is concerned in circumstances which deviate from the aforesaid worst case scenario.
In practice, such known compressors have a speed range with a maximum/minimum rotational speed ratio in the order of magnitude of 2.4, whereas, under favourable conditions, a speed range of 3.2 would be possible.
The present invention aims to remedy the above-mentioned and other disadvantages by providing a compressor with a dynamic speed limiter which automatically maximizes the speed range of the compressor as a function of the operational circumstances, irrespective of the state and condition the compressor is in.
To this aim, the invention concerns an improvement to a compressor of the above-mentioned type which consists in that the compressor is provided with a dynamic speed limiter with what is called a hysteresis module, coupled to the above-mentioned control device of the motor and to the above-mentioned sensors for the outlet temperature and the rotational speed, whereby a hysteresis upper temperature limit and a hysteresis lower temperature limit have been defined in this hysteresis module, as well as an admitted maximum speed range which is determined by a minimum rotational speed and a maximum rotational speed and whereby, as soon as the measured outlet temperature reaches the specified hysteresis upper temperature limit, the actual rotational speed of the compressor element is lowered with a speed jump DS (i.e., an amount of speed change in RPM) when the measured rotational speed is situated in the high speed range close to the maximum rotational speed, or as soon as the measured gas outlet temperature reaches the specified hysteresis lower temperature limit, the actual rotational speed of the compressor element is increased with a speed jump DS when the measured rotational speed is situated in the low speed range close to the minimum rotational speed.
Thanks to the dynamic speed limiter according to the invention, when the aforesaid hysteresis upper temperature limit is reached, which preferably is somewhat lower, for example 2° C. lower than the admitted maximum critical threshold value of the outlet temperature, the rotational speed will automatically be adjusted in the right sense in order to make the outlet temperature decrease.
In this manner, the speed restriction is not determined by a worst case scenario, but under certain favourable circumstances, for example in case of low ambient temperatures, the rotational speed of the compressor will cover the entire speed range which is determined by the limitations of the rotating parts, such that the entire available capacity of the compressor as far as the gas output is concerned can be used completely. Should the circumstances become worse, for example when the ambient temperature rises, the speed range is automatically adjusted as soon as the outlet temperature reaches the aforesaid critical threshold value, such that this threshold value can never be exceeded, not even in case of increasing wear of the compressor.
In the hysteresis module is preferably also defined a hysteresis lower temperature limit whereby, as soon as the measured outlet temperature reaches the specified hysteresis lower temperature limit, the entire aforesaid admitted maximum speed range becomes available again.
This offers the advantage that when the operational conditions of the compressor become more favourable, as a result of which the temperature on the outlet of the compressor element decreases, the capacity of the compressor can be used to the full again.
The invention also concerns a method for compressing a gas whereby a compressor according to the invention is applied. As the operation of the compressor is optimized, there will be less unwanted failures of the compressor.